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Biederman" Subject: Re: [MANPAGE PATCH] Add manpages for move_mount(2) and open_tree(2) To: David Howells References: <153126249579.14533.18370053915999978123.stgit@warthog.procyon.org.uk> <153126248868.14533.9751473662727327569.stgit@warthog.procyon.org.uk> <15449.1531263162@warthog.procyon.org.uk> From: "Michael Kerrisk (man-pages)" Message-ID: Date: Wed, 9 Oct 2019 11:51:44 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <15449.1531263162@warthog.procyon.org.uk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hello David, Your wrote a series of manual pages patches (of which the mail below is one) for the new mount API about a year before the code patches were actually released in the kernel. I'd like to check that these man-pages patches are up to date before merging them. I think they may not be, since there is one patch for fsinfo(2) which does not exist in the kernel, and no manual page for fsconfig(2). I imagine that details may also have changed in the system calls that were ultimately merged. Could you write a manual page for fsconfig(2) please? With respect to the patch below, would you be willing to: * split it into two pieces, one for each page. * review the content to see if it accurately reflects what was merged into the kernel and then resubmit please? Thanks, Michael On 7/11/18 12:52 AM, David Howells wrote: > Add manual pages to document the move_mount and open_tree() system calls. > > Signed-off-by: David Howells > --- > > man2/move_mount.2 | 274 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > man2/open_tree.2 | 260 ++++++++++++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 534 insertions(+) > create mode 100644 man2/move_mount.2 > create mode 100644 man2/open_tree.2 > > diff --git a/man2/move_mount.2 b/man2/move_mount.2 > new file mode 100644 > index 000000000..3a819fb84 > --- /dev/null > +++ b/man2/move_mount.2 > @@ -0,0 +1,274 @@ > +'\" t > +.\" Copyright (c) 2018 David Howells > +.\" > +.\" %%%LICENSE_START(VERBATIM) > +.\" Permission is granted to make and distribute verbatim copies of this > +.\" manual provided the copyright notice and this permission notice are > +.\" preserved on all copies. > +.\" > +.\" Permission is granted to copy and distribute modified versions of this > +.\" manual under the conditions for verbatim copying, provided that the > +.\" entire resulting derived work is distributed under the terms of a > +.\" permission notice identical to this one. > +.\" > +.\" Since the Linux kernel and libraries are constantly changing, this > +.\" manual page may be incorrect or out-of-date. The author(s) assume no > +.\" responsibility for errors or omissions, or for damages resulting from > +.\" the use of the information contained herein. The author(s) may not > +.\" have taken the same level of care in the production of this manual, > +.\" which is licensed free of charge, as they might when working > +.\" professionally. > +.\" > +.\" Formatted or processed versions of this manual, if unaccompanied by > +.\" the source, must acknowledge the copyright and authors of this work. > +.\" %%%LICENSE_END > +.\" > +.TH MOVE_MOUNT 2 2018-06-08 "Linux" "Linux Programmer's Manual" > +.SH NAME > +move_mount \- Move mount objects around the filesystem topology > +.SH SYNOPSIS > +.nf > +.B #include > +.br > +.B #include > +.br > +.B #include > +.br > +.BR "#include " "/* Definition of AT_* constants */" > +.PP > +.BI "int move_mount(int " from_dirfd ", const char *" from_pathname "," > +.BI " int " to_dirfd ", const char *" to_pathname "," > +.BI " unsigned int " flags ); > +.fi > +.PP > +.IR Note : > +There are no glibc wrappers for these system calls. > +.SH DESCRIPTION > +The > +.BR move_mount () > +call moves a mount from one place to another; it can also be used to attach an > +unattached mount created by > +.BR fsmount "() or " open_tree "() with " OPEN_TREE_CLONE . > +.PP > +If > +.BR move_mount () > +is called repeatedly with a file descriptor that refers to a mount object, > +then the object will be attached/moved the first time and then moved again and > +again and again, detaching it from the previous mountpoint each time. > +.PP > +To access the source mount object or the destination mountpoint, no > +permissions are required on the object itself, but if either pathname is > +supplied, execute (search) permission is required on all of the directories > +specified in > +.IR from_pathname " or " to_pathname . > +.PP > +The caller does, however, require the appropriate capabilities or permission > +to effect a mount. > +.PP > +.BR move_mount () > +uses > +.IR from_pathname ", " from_dirfd " and some " flags > +to locate the mount object to be moved and > +.IR to_pathname ", " to_dirfd " and some other " flags > +to locate the destination mountpoint. Each lookup can be done in one of a > +variety of ways: > +.TP > +[*] By absolute path. > +The pathname points to an absolute path and the dirfd is ignored. The file is > +looked up by name, starting from the root of the filesystem as seen by the > +calling process. > +.TP > +[*] By cwd-relative path. > +The pathname points to a relative path and the dirfd is > +.IR AT_FDCWD . > +The file is looked up by name, starting from the current working directory. > +.TP > +[*] By dir-relative path. > +The pathname points to relative path and the dirfd indicates a file descriptor > +pointing to a directory. The file is looked up by name, starting from the > +directory specified by > +.IR dirfd . > +.TP > +[*] By file descriptor. > +The pathname points to "", the dirfd points directly to the mount object to > +move or the destination mount point and the appropriate > +.B *_EMPTY_PATH > +flag is set. > +.PP > +.I flags > +can be used to influence a path-based lookup. A value for > +.I flags > +is constructed by OR'ing together zero or more of the following constants: > +.TP > +.BR MOVE_MOUNT_F_EMPTY_PATH > +.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d > +If > +.I from_pathname > +is an empty string, operate on the file referred to by > +.IR from_dirfd > +(which may have been obtained using the > +.BR open (2) > +.B O_PATH > +flag or > +.BR open_tree ()) > +If > +.I from_dirfd > +is > +.BR AT_FDCWD , > +the call operates on the current working directory. > +In this case, > +.I from_dirfd > +can refer to any type of file, not just a directory. > +This flag is Linux-specific; define > +.B _GNU_SOURCE > +.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed > +to obtain its definition. > +.TP > +.B MOVE_MOUNT_T_EMPTY_PATH > +As above, but operating on > +.IR to_pathname " and " to_dirfd . > +.TP > +.B MOVE_MOUNT_F_NO_AUTOMOUNT > +Don't automount the terminal ("basename") component of > +.I from_pathname > +if it is a directory that is an automount point. This allows a mount object > +that has an automount point at its root to be moved and prevents unintended > +triggering of an automount point. > +The > +.B MOVE_MOUNT_F_NO_AUTOMOUNT > +flag has no effect if the automount point has already been mounted over. This > +flag is Linux-specific; define > +.B _GNU_SOURCE > +.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed > +to obtain its definition. > +.TP > +.B MOVE_MOUNT_T_NO_AUTOMOUNT > +As above, but operating on > +.IR to_pathname " and " to_dirfd . > +This allows an automount point to be manually mounted over. > +.TP > +.B MOVE_MOUNT_F_SYMLINKS > +If > +.I from_pathname > +is a symbolic link, then dereference it. The default for > +.BR move_mount () > +is to not follow symlinks. > +.TP > +.B MOVE_MOUNT_T_SYMLINKS > +As above, but operating on > +.IR to_pathname " and " to_dirfd . > + > +.SH EXAMPLES > +The > +.BR move_mount () > +function can be used like the following: > +.PP > +.RS > +.nf > +move_mount(AT_FDCWD, "/a", AT_FDCWD, "/b", 0); > +.fi > +.RE > +.PP > +This would move the object mounted on "/a" to "/b". It can also be used in > +conjunction with > +.BR open_tree "(2) or " open "(2) with " O_PATH : > +.PP > +.RS > +.nf > +fd = open_tree(AT_FDCWD, "/mnt", 0); > +move_mount(fd, "", AT_FDCWD, "/mnt2", MOVE_MOUNT_F_EMPTY_PATH); > +move_mount(fd, "", AT_FDCWD, "/mnt3", MOVE_MOUNT_F_EMPTY_PATH); > +move_mount(fd, "", AT_FDCWD, "/mnt4", MOVE_MOUNT_F_EMPTY_PATH); > +.fi > +.RE > +.PP > +This would attach the path point for "/mnt" to fd, then it would move the > +mount to "/mnt2", then move it to "/mnt3" and finally to "/mnt4". > +.PP > +It can also be used to attach new mounts: > +.PP > +.RS > +.nf > +sfd = fsopen("ext4", FSOPEN_CLOEXEC); > +write(sfd, "s /dev/sda1"); > +write(sfd, "o user_xattr"); > +mfd = fsmount(sfd, FSMOUNT_CLOEXEC, MS_NODEV); > +move_mount(mfd, "", AT_FDCWD, "/home", MOVE_MOUNT_F_EMPTY_PATH); > +.fi > +.RE > +.PP > +Which would open the Ext4 filesystem mounted on "/dev/sda1", turn on user > +extended attribute support and create a mount object for it. Finally, the new > +mount object would be attached with > +.BR move_mount () > +to "/home". > + > + > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.SH RETURN VALUE > +On success, 0 is returned. On error, \-1 is returned, and > +.I errno > +is set appropriately. > +.SH ERRORS > +.TP > +.B EACCES > +Search permission is denied for one of the directories > +in the path prefix of > +.IR pathname . > +(See also > +.BR path_resolution (7).) > +.TP > +.B EBADF > +.IR from_dirfd " or " to_dirfd > +is not a valid open file descriptor. > +.TP > +.B EFAULT > +.IR from_pathname " or " to_pathname > +is NULL or either one point to a location outside the process's accessible > +address space. > +.TP > +.B EINVAL > +Reserved flag specified in > +.IR flags . > +.TP > +.B ELOOP > +Too many symbolic links encountered while traversing the pathname. > +.TP > +.B ENAMETOOLONG > +.IR from_pathname " or " to_pathname > +is too long. > +.TP > +.B ENOENT > +A component of > +.IR from_pathname " or " to_pathname > +does not exist, or one is an empty string and the appropriate > +.B *_EMPTY_PATH > +was not specified in > +.IR flags . > +.TP > +.B ENOMEM > +Out of memory (i.e., kernel memory). > +.TP > +.B ENOTDIR > +A component of the path prefix of > +.IR from_pathname " or " to_pathname > +is not a directory or one or the other is relative and the appropriate > +.I *_dirfd > +is a file descriptor referring to a file other than a directory. > +.SH VERSIONS > +.BR move_mount () > +was added to Linux in kernel 4.18. > +.SH CONFORMING TO > +.BR move_mount () > +is Linux-specific. > +.SH NOTES > +Glibc does not (yet) provide a wrapper for the > +.BR move_mount () > +system call; call it using > +.BR syscall (2). > +.SH SEE ALSO > +.BR fsmount (2), > +.BR fsopen (2), > +.BR open_tree (2) > diff --git a/man2/open_tree.2 b/man2/open_tree.2 > new file mode 100644 > index 000000000..7e9c86fe3 > --- /dev/null > +++ b/man2/open_tree.2 > @@ -0,0 +1,260 @@ > +'\" t > +.\" Copyright (c) 2018 David Howells > +.\" > +.\" %%%LICENSE_START(VERBATIM) > +.\" Permission is granted to make and distribute verbatim copies of this > +.\" manual provided the copyright notice and this permission notice are > +.\" preserved on all copies. > +.\" > +.\" Permission is granted to copy and distribute modified versions of this > +.\" manual under the conditions for verbatim copying, provided that the > +.\" entire resulting derived work is distributed under the terms of a > +.\" permission notice identical to this one. > +.\" > +.\" Since the Linux kernel and libraries are constantly changing, this > +.\" manual page may be incorrect or out-of-date. The author(s) assume no > +.\" responsibility for errors or omissions, or for damages resulting from > +.\" the use of the information contained herein. The author(s) may not > +.\" have taken the same level of care in the production of this manual, > +.\" which is licensed free of charge, as they might when working > +.\" professionally. > +.\" > +.\" Formatted or processed versions of this manual, if unaccompanied by > +.\" the source, must acknowledge the copyright and authors of this work. > +.\" %%%LICENSE_END > +.\" > +.TH OPEN_TREE 2 2018-06-08 "Linux" "Linux Programmer's Manual" > +.SH NAME > +open_tree \- Pick or clone mount object and attach to fd > +.SH SYNOPSIS > +.nf > +.B #include > +.br > +.B #include > +.br > +.B #include > +.br > +.BR "#include " "/* Definition of AT_* constants */" > +.PP > +.BI "int open_tree(int " dirfd ", const char *" pathname ", unsigned int " flags ); > +.fi > +.PP > +.IR Note : > +There are no glibc wrappers for these system calls. > +.SH DESCRIPTION > +.BR open_tree () > +picks the mount object specified by the pathname and attaches it to a new file > +descriptor or clones it and attaches the clone to the file descriptor. The > +resultant file descriptor is indistinguishable from one produced by > +.BR open "(2) with " O_PATH . > +.PP > +In the case that the mount object is cloned, the clone will be "unmounted" and > +destroyed when the file descriptor is closed if it is not otherwise mounted > +somewhere by calling > +.BR move_mount (2). > +.PP > +To select a mount object, no permissions are required on the object referred > +to by the path, but execute (search) permission is required on all of the > +directories in > +.I pathname > +that lead to the object. > +.PP > +To clone an object, however, the caller must have mount capabilities and > +permissions. > +.PP > +.BR open_tree () > +uses > +.IR pathname ", " dirfd " and " flags > +to locate the target object in one of a variety of ways: > +.TP > +[*] By absolute path. > +.I pathname > +points to an absolute path and > +.I dirfd > +is ignored. The object is looked up by name, starting from the root of the > +filesystem as seen by the calling process. > +.TP > +[*] By cwd-relative path. > +.I pathname > +points to a relative path and > +.IR dirfd " is " AT_FDCWD . > +The object is looked up by name, starting from the current working directory. > +.TP > +[*] By dir-relative path. > +.I pathname > +points to relative path and > +.I dirfd > +indicates a file descriptor pointing to a directory. The object is looked up > +by name, starting from the directory specified by > +.IR dirfd . > +.TP > +[*] By file descriptor. > +.I pathname > +is "", > +.I dirfd > +indicates a file descriptor and > +.B AT_EMPTY_PATH > +is set in > +.IR flags . > +The mount attached to the file descriptor is queried directly. The file > +descriptor may point to any type of file, not just a directory. > + > +.\"______________________________________________________________ > +.PP > +.I flags > +can be used to control the operation of the function and to influence a > +path-based lookup. A value for > +.I flags > +is constructed by OR'ing together zero or more of the following constants: > +.TP > +.BR AT_EMPTY_PATH > +.\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d > +If > +.I pathname > +is an empty string, operate on the file referred to by > +.IR dirfd > +(which may have been obtained from > +.BR open "(2) with" > +.BR O_PATH ", from " fsmount (2) > +or from another > +.BR open_tree ()). > +If > +.I dirfd > +is > +.BR AT_FDCWD , > +the call operates on the current working directory. > +In this case, > +.I dirfd > +can refer to any type of file, not just a directory. > +This flag is Linux-specific; define > +.B _GNU_SOURCE > +.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed > +to obtain its definition. > +.TP > +.BR AT_NO_AUTOMOUNT > +Don't automount the terminal ("basename") component of > +.I pathname > +if it is a directory that is an automount point. This flag allows the > +automount point itself to be picked up or a mount cloned that is rooted on the > +automount point. The > +.B AT_NO_AUTOMOUNT > +flag has no effect if the mount point has already been mounted over. > +This flag is Linux-specific; define > +.B _GNU_SOURCE > +.\" Before glibc 2.16, defining _ATFILE_SOURCE sufficed > +to obtain its definition. > +.TP > +.B AT_SYMLINK_NOFOLLOW > +If > +.I pathname > +is a symbolic link, do not dereference it: instead pick up or clone a mount > +rooted on the link itself. > +.TP > +.B OPEN_TREE_CLOEXEC > +Set the close-on-exec flag for the new file descriptor. This will cause the > +file descriptor to be closed automatically when a process exec's. > +.TP > +.B OPEN_TREE_CLONE > +Rather than directly attaching the selected object to the file descriptor, > +clone the object, set the root of the new mount object to that point and > +attach the clone to the file descriptor. > +.TP > +.B AT_RECURSIVE > +This is only permitted in conjunction with OPEN_TREE_CLONE. It causes the > +entire mount subtree rooted at the selected spot to be cloned rather than just > +that one mount object. > + > + > +.SH EXAMPLE > +The > +.BR open_tree () > +function can be used like the following: > +.PP > +.RS > +.nf > +fd1 = open_tree(AT_FDCWD, "/mnt", 0); > +fd2 = open_tree(fd1, "", > + AT_EMPTY_PATH | OPEN_TREE_CLONE | AT_RECURSIVE); > +move_mount(fd2, "", AT_FDCWD, "/mnt2", MOVE_MOUNT_F_EMPTY_PATH); > +.fi > +.RE > +.PP > +This would attach the path point for "/mnt" to fd1, then it would copy the > +entire subtree at the point referred to by fd1 and attach that to fd2; lastly, > +it would attach the clone to "/mnt2". > + > + > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.\""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""" > +.SH RETURN VALUE > +On success, the new file descriptor is returned. On error, \-1 is returned, > +and > +.I errno > +is set appropriately. > +.SH ERRORS > +.TP > +.B EACCES > +Search permission is denied for one of the directories > +in the path prefix of > +.IR pathname . > +(See also > +.BR path_resolution (7).) > +.TP > +.B EBADF > +.I dirfd > +is not a valid open file descriptor. > +.TP > +.B EFAULT > +.I pathname > +is NULL or > +.IR pathname > +point to a location outside the process's accessible address space. > +.TP > +.B EINVAL > +Reserved flag specified in > +.IR flags . > +.TP > +.B ELOOP > +Too many symbolic links encountered while traversing the pathname. > +.TP > +.B ENAMETOOLONG > +.I pathname > +is too long. > +.TP > +.B ENOENT > +A component of > +.I pathname > +does not exist, or > +.I pathname > +is an empty string and > +.B AT_EMPTY_PATH > +was not specified in > +.IR flags . > +.TP > +.B ENOMEM > +Out of memory (i.e., kernel memory). > +.TP > +.B ENOTDIR > +A component of the path prefix of > +.I pathname > +is not a directory or > +.I pathname > +is relative and > +.I dirfd > +is a file descriptor referring to a file other than a directory. > +.SH VERSIONS > +.BR open_tree () > +was added to Linux in kernel 4.18. > +.SH CONFORMING TO > +.BR open_tree () > +is Linux-specific. > +.SH NOTES > +Glibc does not (yet) provide a wrapper for the > +.BR open_tree () > +system call; call it using > +.BR syscall (2). > +.SH SEE ALSO > +.BR fsmount (2), > +.BR move_mount (2), > +.BR open (2) > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/